The present invention relates generally to equipment utilized and operations performed in conjunction with wireless telemetry and, in an embodiment described herein, more particularly provides thermal expansion matching for an acoustic telemetry system used with a subterranean well.
In order to stabilize a stack of electromagnetically active elements (such as piezoceramic, electrostrictive or magnetostrictive discs or rings) during transport and handling, thereby preventing damage to the elements, a compressive force is typically applied to the elements. The compressive force also operates to bias the elements against a transmission medium (such as a tubular string in a well), thereby ensuring adequate acoustic coupling between the transmission medium and the elements.
To prevent the compressive force from being reduced or even eliminated as temperature increases (due to the fact that the elements generally have a coefficient of thermal expansion which is much less than a housing in which the elements are contained), various methods have been proposed which attempt to equalize the compressive force over a range of temperature variation. In these methods, the compressive force remains substantially constant (or even increases somewhat) as the temperature increases.
However, there are several problems with these prior methods. For example, these methods are not able to take advantage of the fact that most electromagnetically active elements are less susceptible to compressive depolarization at reduced temperatures. Thus, more compressive force may be satisfactorily applied to an electromagnetically active material as temperature decreases, providing enhanced protection from damage during handling. As another example, efforts directed at providing a substantially constant compressive force have resulted in increased assembly lengths, which in turn increases the cost and decreases the convenience of utilizing these methods.